Door lock

ABSTRACT

A door lock in which there is provided a latch bolt, a dead bolt, a first lever secured to the dead bolt and having an abutting portion being capable of abutting against the rear portion of the latch bolt, a second lever pivotably connected at one end to the first lever, abutting means on the second lever and being capable of abutting against the rear portion of the dead bolt, and a third lever pivotally connected at one end to the other end of said second lever and rockable about its axis. The second lever and the third levers move in a manner of toggle levers when the third lever is rocked about its axis and, when the dead bolt is in its projected position, the first, second and third levers are disposed in such a manner that the first lever abuts against the rear surface of the latch bolt and an external force acting directly on the dead bolt to retract the same will tend to prevent rotation of the second lever and the third lever by abutting engagement of the abutting means with the rear portion of the dead bolt whereby both the latch bolt and the dead bolt are restrained against retraction.

United States Patent [191 Wada [451 Jan. 8, 1974 1 DOOR LOCK [76] Inventor: Waichi Wada, 35-29, l-chome,

Ilkegami, Ohta-ku, Tokyo, Japan [22] Filed: Feb. 23, 1972 [21] Appl. No.: 228,713

[56] References Cited UNITED STATES PATENTS l/l919 Weller .Q 70/107 8/1928 Heyer 70/110 6/1961 Primary ExaminerMarvin A. Champion Assistant Examiner-Richard P. Tremblay Att0rneyJohn C. Holman et al.

Kuo 292/DIG. l5

[5 7 ABSTRACT A door lock in which there is provided a latch bolt, a dead bolt, a first lever secured to the dead bolt and having an abutting portion being capable of abutting against the rear portion of the latch bolt, a second lever pivotably connected at one end to the first lever, abutting means on the second lever and being capable of abutting against the rear portion of the dead bolt, and a third lever pivotally connected at one end to the other end of said second lever and rockable about its axis. The second lever and the third levers move in a manner of toggle levers when the third lever is rocked about its axis and, when the dead bolt is in its projected position, the first, second and third levers are disposed in such a manner that the first lever abuts against the rear surface of the latch bolt and an external force acting directly on the dead bolt to retract the 4 Claims, 5 Drawing Figures I 30 JEI I I 28b 27b PATENTEU 81974 3.783.658

sum 1 BF 4 FBGI PRIOR ART PATENTED 8'974 3.783.658

sum 2 or 4 PAIENTEDJAN 81974 I 3,788,658

SHEET 3 OF 4 PAIENIEUJAN 6:914 3.788.658

SHEET u 0F 1 FIG. 4-

DOOR LOCK BACKGROUND OF THE INVENTION This invention relates to a door lock and, more particularly, to a locking mechanism for a door lock comprising a latch bolt and a dead bolt;

There is a certain standard in what persons skilled'in the art calls throw which is the length of the portion of a dead bolt projecting from the outer surface of a front plate of the door lock. Currently, the standard throw is 3/4 inch. There has been a recent tendency, however, for the throw to become longer and the standard of three-fourths inch throw is gradually being replaced by I inch throw. Such lengthening of the throw is certainly desirable from the standpoint of security but it poses a problem in the design of a deadbolting mechanism in a door lock. Inasmuch as a present standard of distance between the outer surface of the front plate and the axis of a knob is 2% inches, it is rather difficult according to constructions of prior art door locks to accommodate the dead bolt having a prolonged throw of 1 inch together with a mechanism comprising levers and other elements required for reciprocating the dead bolt within the inside space of the door lock.

PRIOR ART For instance, a mechanism for moving the dead bolt between its projected and retracted positions in a prior art door lock comprises, as shown in FIG. 1, a dead bolt 3 having a three-fourths inch throw, a lever 4 secured to the dead bolt 3 and formed with an oblique slot 5 and an actuating lever 6 which engages the oblique slot 5. FIG. 1 is an elevational view of the prior art door lock 1 with its upper plate 2 partly cut away. According to this prior art mechanism, the foremost end portion 6a of the actuating lever 6 has a projection which engages the oblique slot 5 of the lever 4 secured to the dead bolt 3. The actuating lever 6 is rotated by known means such as a key-actuated tang 7. When the end portion 6a is at the upper end of the oblique slot 5, the dead bolt 3 is in the retracted position. As the actuating lever 6 is rotated counterclockwise, the end portion 6a slides along the slot 5 thereby causing the dead bolt 3 to'move forward to its projected position. When the end portion 6a reaches the lower end of the slot 5 as shown in FIG. l, the dead bolt 3 reaches its foremost projected position. As will be apparent from the figure, the throw of the dead bolt 3 is determined by thedistance of the travel of the end portion 6a of the actuat ing lever 6 in the oblique slot 5 and proper angle of which relative-to a horizontal line is about 45 degrees.

It will be understood that the upper end of the slot 5 is nearly adjacent to the inner surface of the front. plate 8 when the dead bolt 3 is in the projected position and further obliquely upward extension of the slot 5 is practically impossible. Likewise, the lower end ofthe slot 5 is nearly adjacent to the hub portion of the actuating lever 6 when the dead bolt 3 is'in the retracted position so thatfurther obliquely downward extension of the slot 5 is also impossible. In other words, the conventional three-fourths inch throw is the maximum throw which can be obtained by this particular construction. Accordingly, this type of deadbolting mechanism en'-' counters difficulty in meeting the current requirement for extending the throw to 1 inch.

Further, ithas been a common practice in the art to provide some stop means for restraining a dead bolt against retraction to prevent unlocking of the door lock for unlawful purposes by applying an external force directly to the dead bolt to forcibly retract the same. It is, however, not sufficient from the standpoint of security to provide such stop means only for the dead bolt because in this case the latch bolt can be easily moved, for retracted by applying an external force thereto by means, for example, of a screw driver. It is, accordingly, desirable from the standpoint of security not only to extend the throw but also to provide means for simultaneously'restraining retraction of both the dead ,bolt and the latch bolt. There are some prior art door locks in which such simultaneous locking means for the dead bolt and the latch bolt are provided. However, such prior art locking means are not appropriate or advantageous when used in combination with the mechanism for lengthening the throw according to the invention which will be described hereinbelow.

OBJECTS AND SUMMARY OF THE INVENTION It is, therefore, a general object of the invention to provide a novel and useful door lock which eliminates the above described disadvantages of the prior art door locks.

It is another object of the invention to provide a door lock in which a dead bolt of a longer throw is obtainable without enlarging the casing or extending the standard distance between the front plate and the axis of a knob. According to the present invention, the dead bolt of a longer throw can be obtained by employing a pair of levers which are pivotably connected to each other and cooperate with each other in the manner of toggle levers to move the dead bolt between its two extreme positions.

It is another object of the invention to provide a door lock in which both a dead bolt and a latch bolt are simultaneously restrained against retraction by a very simple mechanism when they are in their projected positions. The dead bolt is restrained against retraction by abutting means which are provided on one of said pair of levers arranged nearer to the dead bolt and are brought into a direct abutting engagement with the rear portion of the dead bolt when an external force applied to the dead bolt tends to rotate the pair of levers. The abutting means are advantageous in that they do not require a plurality of parts which cooperate with each other in a complicated manner to restrain the dead bolt against retraction. Such mechanism comprising a plurality of parts cooperating with each other would naturally be more expensive and moreover would fail to prevent the retraction of the dead bolt if only one of the parts becomes defective, whereas the abutting means according to the invention is cheap and less likely to cause failure because of its simple constructionand a direct engagement with the rear portion of the dead bolt. In the meanwhile, the latch bolt is restrained against retraction by abutting engagement of a portion ,of another lever secured to the dead bolt with the latch Additional objects and features of the invention will become apparent from the description set forth hereafter when considered in conjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of a prior art door lock with its upper plate partly cut away;

FIG. 2 is an elevational view of a door lock, with its upper plate removed, which embodies the present invention, shown in a state in which the dead bolt is in a retracted position;

FIG. 3 is a perspective of retractor hubs used for retracting the latch bolt;

FIG. 4 is a view similar to FIG. 2 but shown in a state in which the dead bolt is in a projected position; and

FIG. 5 is an elevational view of one modification of abutting means employed for preventing retraction of the dead bolt.

DETAILED DESCRIPTION OF THE INVENTION Referring now more particularly to the drawings, a door lock to which the present invention is applicable is designated generally by reference character L. This lock is of standard size as will be well understood by those skilled in the art. In FIG. 2, a case designated generally by reference numeral comprises a box-like element 11 with which is associated a cover plate, which is not shown in the figure for clarity of illustration, to define a chamber having an open edge 12. A front plate 13 carries lugs 14, adapted to be received in the chamber through the open edge 12 for engagement by screws 15, which retain the front plate 13 in closing relationship to the open edge 12. The front plate 13 is formed with an opening 16. A head 17a of a latch bolt 17 is guidingly received in the opening 16. A stem 18 of the latch bolt 17 has a stop 19 rigidly secured to the proximal end thereof. The stop 19 has a forked end 19a which straddles over the horizontal portion of a lever 20 which will be described more in detail later. There are also provided guide bosses 21 and 22 on floor 11a of the box-like element 11 for guiding the stem 18. Thus, the stem 18 is guided in the opening 16, the guide bosses 21 and 22 and along the horizontal portion of the lever 20 for reciprocation on its own axis whereby the head 17a may be projected or retracted through the opening 16. An annular spring bearing 23 is loosely fitted on the stem 18. A compression spring 24 is provided for the stem 18 between the spring bearing 23 and the rear surface of the latch bolt head 17a. The compression spring 24 urges, on one hand, the spring bearing 23 against the guide bosses 21 and 22 and, on the other hand, the head 17a to project from'the front plate 13. The latch bolt 17 is restrained against its forward movement by engagement of the stop 19 fixed theretowith the rear edge of a lever 25 to be described later.

Nextly, a mechanism for actuating the latch bolt 17 will be described. The actuator mechanism comprises a first actuator 26, a second actuator 27, a first retractor hub 28, a second retractor hub 29, and a leaf spring 30. The first actuator 26 is pivotally mounted on pivot 31 which is disposed at the right bottom comer of the case 10. The first actuator 26 has an extended arm portion 26a which is in abutting engagement with the stop 19, when the latch bolt 17 is in its projected position and thereby is cooperable with the stop 19 to retract the latch bolt 17. The first actuator 26 is formed with a slot 32 for engagement with a pin 27a of the second actuator 27 as will be described later.

The actuator 26 is also provided with an uplift 26b in the middle portion of one edge thereof, the function of which will be described later. The second actuator 27, pivotally mounted on a pivot 33 provided at a position above the pivot 31 in the case 10, is disposed under the first actuator 26. The second actuator 27 has the pin 27a which is rigidly secured thereto in the middle portion thereof and slidably engages in the slot 32 formed in the first actuator 26. The second actuator 27 is provided with an uplift 27b at the foremost end thereof. The first retractor hub 28 is mounted in the case 11 for oscillation about an axis 34. The first retractor hub 28 is formed with a square opening 28a for receiving the spindle of an inside knob (not shown). The first retractor hub 28 has two winged portions 28b and 280. The second retractor hub 29 is also mounted in the case 10 for oscillation about the axis 34. The second retractor hub 29 is formed with the square opening 29a-as shown in FIG. 3 for receiving the spindle of an outside knob (not shown). The second retractor hub 29 has two winged portions 29b and 29c. These two hubs are conventional and are adapted to lie one against the other. The leaf spring 30 which is substantially of a U-shape is supported at one end thereof on a projection 35 formed integrally with the box-like element 11 and is wound around the pivot 31 at its curved portion. The other end of the leaf spring 30 constantly bears against the pin 27a of the second actuator 27 and urges both the first and second actuators 26 and 27 against the first and second retractor hubs 28 and 29 through the pin 27a. When the first retractor hub 28 is rotated about the axis 34 in a clockwise direction, the vertical end surface of the winged portion 280 pushes the uplift 26b of the first actuator 26 thereby rotating the first actuator 26 about the pivot 31 in a clockwise direction against the force of the leaf spring 30 to retract the latch bolt 17. When the first retractor hub 28 is rotated about the axis 34 in a counterclockwise direction, the vertical end surface of the winged portion 28b pushes the uplift 27b of the second actuator 27 thereby rotating the second actuator 27 about the pivot 33 in a counterclockwise direction against the force of the leaf spring 30. .As the second actuator 27 rotates in a counterclockwise rotation, the pin 27a of second actuator 27 which is engaged in the slot 32 in the first actuator 26 pushes the first actuator 26, while sliding in the slot 32 from one extreme position to another thereby rotating the actuator 26 about the pivot 31 in a clockwise direction to retract the latch bolt 17. Thus, the first retractor hub 28 is effective, upon rotary movement in either direction about the axis 34, to move the first actuator 26b for retracting the latch bolt 17. The second retractor hub 29 is likewise so constructed and arranged that, upon rotary movement in either direction about the axis 34, it will cause clockwise movement of the first actuator 26 for retracting the latch bolt 17.

It will be apparent from FIG. 3 that the second retractor hub 29 extends in a lateral direction beyond the first retractor hub 29 so as to allow for the forming of a locking slot 36. This locking slot cooperates with a stop unit designated generally by reference numeral 37. The stop unit 37 comprises a pair of buttons 38,39, levers 40, 41 respectively connected to the buttons 38,39 and a lever 42 interconnecting the levers 40 and 41.

lever 51 is formed in the-middle portion thereof with a square opening 51b through which the spindle of a element 11 for guiding the levers 40 and 41 in their reciprocating sliding movement. A spring 47 is supported at one end thereof between the guide boss 22 and a guide boss 48 and, at the other end, bears against the end 42a of the lever 42 so as to hold the lever 42 against rotation and thereby prevent an unintended sliding movement of the levers 40 and 41. Operation of the buttons 38, 39 brings the lever 41 into or out of the locking-slot 36 thereby locking or releasing the second retractor hub 29. When the second retractor hub 29 is locked, it cannot rotate to retract the latch bolt 17. Ac- 7 cordingly, the outside knob which is connected to the second retractor hub 29 through its spindle is locked or released by the operation of the buttons 38 and 39. Even when the second retractor hub 29 is locked, the first retractor hub 28 is free to rotate so that the inside knob connected to the first retractor hub 28 through its spindle is operable to retract the latch bolt 17.

The locking mechanism embodying the invention comprises a dead bolt 49, the first lever 20, a second lever 50, a third lever 51, abutting means for preventing retraction of the dead bolt 49 which consist of a pawl portion 50b of the second lever 50 and rocking means for rocking the third lever about its fixed axis both from outside and inside the door including a cylinder lock assembly 53.

The front plate 13 is formed with another opening I 54. The dead bolt 49 is guidingly received in the opening 54 for reciprocation. The first lever 20 is substantially of an L-shape and consists of a vertical portion 20a and a horizontal portion 20b. The vertical portion 20a of the first lever 20 is rigidly securedto theirear end portion of the dead bolt 49 by means of rivets 55. The horizontal portion 20b is slidably received in' a space between the guide boss21 and a boss56 which I is formed integrally with the box-like element 11 for rethe first lever 20 is guided by the guide boss 21 and the boss 56 for reciprocation in a horizontal direction wherebythe dead bolt. 49 may be projecte'd orretracted' through the opening '54. The first lever-20 is also provided with a lug 200 in the horizontal portion 20b. The lug'20c is bent toward thefloor of the box-like element 11 for abutting engagement with a lever 57 to be described later- The lowerfront end portion of thetion 20a of the lever'20 by means of a pin'66. The second lever 50 is formed in the other end portion thereof with a hole 50a. The second lever 50 has a pawlportion 50b at theforemost end thereof. The lever '50 is so dis- 50b abuts against the rear portion of the lower surface of the dead bolt 49 when the dead bolt 49 is in its projected position. v I

The third lever-51 has a sleeve portion 51a which is rotatably fitted in the opening of the boss 56. The third l ceiving the sleeve portion of the third lever 51. Thus i 37 and therefore the outside knob is not thumb turn (not shown) provided inside the door is inserted. Therefore, the axis of rotation of the third lever 51 extends through the opening 51b. The third lever 51 carries at one endthereof nearer to the dead bolt 49 a pin 510 which engages in the opening 50a of the lever 50 for pivotal connection therewith. The third lever 51 further has two projections 51d and 51e. The box-like element 11 is formed in its floor portion with a threaded opening 58 with which the cylinder lock assembly 53 is in threaded engagement. The cylinder lock assembly 53 which is of a conventional type has a key-actuated tang 59. In the stage shown in FIG. 2 in which the dead bolt 49 is in the retracted position, the tang 59 is disposed in the space between the two projections 51d and Sle, being ready for pushing the projection 51d and thereby rotating the third lever 51 in a counterclockwise direction. The cylinder lock assembly 53 is held against rotation in the threaded opening 58 by a screw 60 which extends through the front plate 13 and engages in an axial slot 61 formed in the cylinder lock assembly 53. I i

v A leaf spring 52 which is substantially of a U-shape is supported at one end thereof on a projection ,62 which is integrally formed with the box-like element 1 1 and is wound around a pin 63 formed integrally with the box-like element 11. In the state shown in FIG. 2, the other end of the leaf spring 52 bears against the upper edge portion of said end of the third lever 51 remote from the dead bolt 49.

The lever 57 which is of ajshoe-like configuration is pivotally mounted on the floor of the box-like element 11 about a pin 64.-The lever 57 has two lugs 57a and 57b. The lug 57a is engageable with the tang 59,

whereas the lug 57b is engageable with a pin 25a turned in a counterclockwise directionfrom ,the position shown by adotted line in FIG.v 2, the tang 59 pushes the lug 57d and thereby causes the lever 57 to rotate'in a clockwise-direction. The clockwise rotation of the lever 57 causes its lug 5 7bto push the'pin 25a of the lever 25 downwardly. As the pin ZSaispushed downwardly, the lever 25, rotates in a counterclockwise direction thereby retracting the latch bolt 17. Thus, these levers 57 and 25 cooperate with each other to retract the latch bolt 17. It will be noted that this mechanism is effective for opening the door from outside when the second retractor hub 29 is locked by the stop available for retracting the latch bolt 17. i

Nextly, the operation of the locking mechanism according to the invention will be described. i

In the stateshown in FIG. 2, the deadbolt 49 is in its retracted position. The first lever 20 is in its innermost position with its rear end abutting against-a lug 65 of the'box-like element 11. The second lever 50 is nearly in an uplifted position with its end connecting tothe posed that the upper surface 50c of the pawl portion third lever 51 raised and its pawl portion 50b disengaged from the lower rear corner of the dead bolt 49. The'third lever 51 is in its one extreme position'with itsend nearer to the dead bolt 49 raised and the end re- 'mote from the dead bolt 49 lowered. It will be noted that in this position the connecting point at which the second lever 50 and the third lever 51 are pivotably connected to each other is substantially deviated on one side of the line connecting the center of the pin 66 and the axis of the third lever 51. The leaf spring 52 which bears against the upper edge portion of said end of the third lever 51 remote from the dead bolt 49 tends to rotate the third lever 51 in a clockwise direction thereby transmitting a force to the dead bolt 49 through the second lever 50 and the first lever in a direction to retract the dead bolt 49. When the tang 59 is turned by means of the key in a clockwise direction, i.e., in the direction of arrow A from the position shown in full lines in FIG. 2, the tang 59 engages the projection 51d and causes the third lever 51 to rock in a counterclockwise direction against the force of the leaf spring 52. As the third lever 51 is rocked in the counterclockwise direction, the second lever 50 which is operatively connected to the third lever 51 pivots about the pin 66 in a clockwise direction, causing the first lever 20 and the dead bolt 49 secured thereto to slide to the left as viewed in the drawing, whereby the dead bolt 49 is brought to its projected position as shown in FIG. 4. As the third lever 51 is rocked from its one extreme position shown in FIG. 2 to the other extreme position shown in FIG. 4, its end remote from the dead bolt 49 is raised and the leaf spring 52 which bore against the upper edge portion of said end in the first extreme position now bears against the lower edge of said end. Accordingly, the leaf spring 52 now tends to rotate the third lever 51 in a counterclockwise direction. It will be observed'in FIG. 4 that the second lever 50 and the third lever 51 are disposed nearly in a straight alignment. It will be understood that a relatively long throw of the dead bolt 49 is obtainable by the locking mechanism .according to the invention in which the throw is the difference between the maximum distance between the pivot 66 and the axis of the third lever 51 which is obtained when the levers 50 and 51 are' nearly in a straight alignment and the minimum distance between said pivot and said axis which is obtained when the two levers are rotated upwardly to the point at which the rear end of the lever 20 abuts against the lug 65 and further retraction of the dead bolt 49 is impossible. Since the distance between said pivot and said axis can be minimized, if so designed, to a point at which the second lever 50 is in close proximity-to the sleeve portion of the third lever 51, a maximum available throw of the dead bolt 49 is only slightly shorter than the distance between the front plate 13 and the axis of the third lever 51 and obviously larger than the maximum throw of the previously described 'mechanism of the prior art which is determined by the travel 'of the end of the actuating lever in the oblique slot.

When the dead bolt 49 is in its projected position, the upper surface 50c of the pawl portion 50b of the second lever 50 is in abutting engagement with the lower surface of the rear portion of the dead bolt 49. It is to be noted that when the dead bolt 49 is in its projected position, the second lever 50 and the third lever 51 are disposed in a position slightly beyond the dead center 49. It will be understood from the foregoing description that retraction of the dead bolt 49 by an external force can be prevented by the hinge connection of the second and the third levers 50 and 51 and the cooperation between the pawl portion 50b of the second lever 50 and the dead bolt 49 inasmuch as the second lever 50 and the third lever 51 are disposed substantially in a position slightly beyond the dead center position relative to these levers 50 and 51 in a direction in which an external force applied to the dead bolt 49 will tend to prevent the rotation of these levers 50 and 51 by engagement of the pawl portion 50 b of the lever 50 with the dead bolt 49. In the present embodiment, the leaf spring 52 serves to apply a force to the second and third levers 50 and 51 which force tends to rotate these levers beyond the dead center position relative thereto and urge the upper surface 50c of the pawl rotation 50b against the lower surface of the dead bolt 49. Thus, the leaf spring 52 is effective to prevent retraction of the dead bolt 49 due to the external force applied directly to the same.

When the dead bolt 49 is brought to its projected position, the lower front surface of the lever 20 abuts against the upper rear surface of the latch bolt 17 and holds the same againstretraction.

The foregoing is the operation of the locking mechanism when the third lever 51 is actuated by the tang 59 by means of a key inserted in the cylinder lock assembly 53 from outside the door. It will be apparent, however, that the third lever 51 can also be rocked by the thumb turn provided inside the door. Accordingly, the third lever 51 is rockable by the rocking means which are accessible both from outside and inside the door.

For retracting the dead bolt 49, the tang 59 is turned in a counterclockwise direction. The third lever 51 is rocked from the extreme position shown in FIG. 4 to the extreme position shown in FIG. 2. The second lever 50 is pivoted in a counterclockwise direction causing the first lever 20 and the dead bolt 49 to move to the right. The leaf spring 52 now bears against the upper edge of the end portion of the third lever 51 thereby applying force to rotate it in a clockwise direction. Accordingly, the leaf spring 52 serves to retract the dead bolt 49 quicklyto the position shown in FIG. 2. After the dead bolt 49 is retracted, the leaf spring 52 acts on the third lever 51 to hold it in position. Accordingly,

-unintended projection of the dead bolt 49 due to a shock given to the door or other causes is prevented.

In FIG. 5, an alternative form of the means for engaging with the rear portion of the dead bolt 49 is shown. The dead bolt 49 is formed with a recess 49a in the lower rear portion thereof and the foremost end 50b of the second lever 50 engages in the recess 49a when the dead bolt 49 is in its projected position. It will be readily understood that a force applied to the dead bolt 49 to retract the same tends to rotate the second lever 50 in a clockwise direction thereby causing the upper surface of the end portion 50b to bear against the lower surface 49b of the rear portion of the dead bolt 49.

What I claim is:

l. A door lock comprising a'latch bolt, 9. dead bolt, a first lever secured to said dead bolt and having an abutting portion capable of abutting against the rear portion of said latch bolt, a second lever pivotably connected at one end to said first lever, abutting means provided on said second lever and capable of abutting against the lower surface of the rear portion of said dead bolt, a third lever pivotably connected at one end nearer to said dead bolt to the other end of said second 'lever and rockable about its axis between a first extreme position and a second extreme position, said first extreme position corresponding to a retracted position of said dead bolt in which the connecting point where said second lever and said third lever are pivotably connected to each other is substantially deviated on one side of a line connecting the point at which said second lever is pivotably connected to said first lever and said axis of said third lever and said second extreme position corresponding to a projectedposition of said dead bolt in which said connecting point is slightly deviated on the other side of said line, and actuating means for rockingsaid third lever, each of said first, second and third levers being disposed, when said third lever is in said second extreme position, in such a manner that said first lever abuts in said abutting portion against the rear surface of said latch bolt, said abutting means abuts against the lower surface of the rear portion of said dead bolt so that an external force acting directly on said dead bolt to retract the same will tend to prevent rotation of said second lever and said third lever by abutting engagement of said abutting means with the rear portion of said dead bolt whereby both said latch bolt and said dead bolt are restrained against retraction.

2. The door lock as defined in claim 1 in which said abutting means is a pawl portion formed at the end of said second lever nearer to said dead bolt, a horizontal surface of said pawl portion being engageable with the opposing lower horizontal surface of the rear portion of said dead bolt.

thumb-turn accessible from inside the door. 

1. A door lock comprising a latch bolt, a dead bolt, a first lever secured to said dead bolt and having an abutting portion capable of abutting against the rear portion of said latch bolt, a second lever pivotably connected at one end to said first lever, abutting means provided on said second lever and capable of abutting against the lower surface of the rear portion of said dead bolt, a third lever pivotably connected at one end nearer to said dead bolt to the other end of said second lever and rockable about its axis between a first extreme position and a second extreme position, said first extreme position corresponding to a retracted position of said dead bolt in which the connecting point where said second lever and said third lever are pivotably connected to each other is substantially deviated on one side of a line connecting the point at which said second lever is pivotably connected to said first lever and said axis of said third lever and said second extreme position corresponding to a projected position of said dead bolt in which said connecting point is slightly deviated on the other side of said line, and actuating means for rocking said third lever, each of said first, second and third levers being disposed, when said third lever is in said second extreme position, in such a manner that said first lever abuts in said abutting portion against the rear surface of said latch bolt, said abutting means abuts against the lower surface of the rear portion of said dead bolt so that an external force acting directly on said dead bolt to retract the same will tend to prevent rotation of said second lever and said third lever by abutting engagement of said abutting means with the rear portion of said dead bolt whereby both said latch bolt and said dead bolt are restrained against retraction.
 2. The door lock as defined in claim 1 in which said abutting means is a pawl portion formed at the end of said second lever nearer to said dead bolt, a horizontal surface of said pawl portion being engageable with the opposing lower horizontal surface of the rear portion of said dead bolt.
 3. The door lock as defined in claim 1 in which said dead bolt is formed with a recess at a rear corner thereof and said abutting means is the end portion of said second lever nearer to said dead bolt which is engageable with said recess formed in said dead bolt.
 4. The door lock as defined in claim 1 in which said actuating means are a key-actuated tang of a cylinder lock assembly accessible from outside the door and a thumb-turn accessible from inside the door. 